Young children tend to rely on reactive cognitive control (e.g. strongly slow down after an error), even when task accuracy would benefit from proactive cognitive control (taking a slower task approach up front). We investigated if giving young primary school children opportunities to repeatedly experience tasks where success rates depend on balancing speed and accuracy by using proactive and reactive cognitive control promotes their selective use of proactive cognitive control strategies. Participants were children from the German-speaking part of Switzerland (N = 105; mean age 7.5 yrs). They were allocated to a "training with feedback," "training without feedback," or control condition. They completed a pre- and posttest (Hearts and Flower task), separated by six computerized training tasks over three weeks. We analyzed time-by-condition effects on reaction time, rate-correct, and post-error slowing. In line with our hypotheses, both training groups outperformed the control group on reaction time and rate-correct in the incongruent block. Contrary to expectation, feedback did not enhance the intervention effect and the training group without feedback additionally outperformed control participants on rate-correct in the mixed block and improved their post-error slowing. Thus, our short intervention triggered accelerated development of proactive control in a cognitive conflict task requiring accuracy-speed trade-offs. This shows that during the developmental period in which children are assumed ready for developing proactive cognitive control strategies, repeated experiences over a short time-period can lead to acute improvements. More research is needed to check for long-term benefits and generalizability beyond our sample and the tasks employed.